"The Environmental Protection Agency said on Tuesday it has issued a subpoena to Halliburton for not revealing information about liquids used in a natural gas drilling technique called 'fracking.'

In September, the EPA had asked nine companies that practice hydraulic fracturing, or fracking, to reveal the mix of chemicals they use in the practice which is opposed by environmental groups worried about its effect on drinking water.

The EPA says it needs the data on fluids used in fracking to complete its comprehensive study of the technique. During fracking, companies inject millions of gallons of water, sand and chemicals as far as two miles underground to break open fissures in the gas-bearing shale.

The EPA is slated to release preliminary results of the study, commissioned by Congress, by the end of 2012.'